Fiber-reinforced resin materials (FRPs) obtained by impregnating reinforcement fibers with a matrix resin have conventionally been lighter than metal materials and stronger than resin materials. In particular, those that use a thermoplastic resin for the matrix resin are receiving attention for applications in mechanical products, e.g., vehicle body parts, etc., due to the ease with which they can be molded.
However, when a thermoplastic resin is used for the matrix resin, since thermoplastic resins are more viscous than thermosetting resins, it is not easy to impregnate gaps in the reinforcement fibers with the thermoplastic resin. As such, by way of example, there is a practice where impregnation with a thermoplastic resin is performed by making the thickness of a bundle of reinforcement fibers that are drawn and aligned in the same direction thinner by spreading a multifilament, and so forth. However, since a fiber-reinforced resin sheet comprising a reinforcement fiber and a thermoplastic resin becomes thin, such sheets are laminated and integrally molded to obtain the desired thickness.
By way of example, in the fiber-reinforced resin material production process of Patent Literature 1, first, a plurality of reinforcement fiber sheets, in which a plurality of reinforcement fibers are drawn and aligned in the same direction, and thermoplastic resin reinforcement sheets each comprising a thermoplastic resin sheet are laminated. Next, in order to position these sheet materials, the sheet materials are fixed to one another with a stitching thread made of a thermoplastic resin. By thereafter heating and pressurizing the laminated sheet materials, a fiber-reinforced resin material is produced.
With such a production process, since the laminated sheets are integrated with one another with the stitching thread, sheet misalignments are unlikely to occur, and operability is also favorable. Further, by virtue of the fact that the stitching thread comprises a thermoplastic resin, this stitching thread impregnates the gaps between the reinforcement fibers at the time of molding.
In addition, in Patent Literature 2, there is proposed a process for producing a multi-axially laminated reinforcement fiber sheet (fiber-reinforced resin material) in which a reinforcement fiber sheet, which is drawn and aligned in the same direction, and an oblique reinforcement fiber sheet, which has an orientation angle that differs from the fiber direction of the reinforcement fiber sheet, are laminated with a thermoplastic resin matrix layer in-between, and the these layers are bonded with one another with a thermal adhesive.
Further, in Patent Literature 3, there is proposed a process for producing a fiber-reinforced resin material in which ingredient sheets comprising a reinforcement fiber and a thermoplastic resin are laminated, and the sheets are thermally bonded with one another through ultrasonic spot welding.